Showing papers on "Seebeck coefficient published in 2017"

Large anomalous Nernst effect at room temperature in a chiral antiferromagnet

Abstract: The anomalous Nernst effect is usually associated with ferromagnets — enabling a temperature gradient to generate a transverse electric field — but the Berry curvature associated with Weyl points can drive this phenomenon in chiral antiferromagnets. A temperature gradient in a ferromagnetic conductor can generate a transverse voltage drop perpendicular to both the magnetization and heat current. This anomalous Nernst effect has been considered to be proportional to the magnetization1,2,3,4,5,6,7, and thus observed only in ferromagnets. Theoretically, however, the anomalous Nernst effect provides a measure of the Berry curvature at the Fermi energy8,9, and so may be seen in magnets with no net magnetization. Here, we report the observation of a large anomalous Nernst effect in the chiral antiferromagnet Mn3Sn (ref. 10). Despite a very small magnetization ∼0.002?μB per Mn, the transverse Seebeck coefficient at zero magnetic field is ∼0.35?μV?K−1 at room temperature and reaches ∼0.6?μV?K−1 at 200?K, which is comparable to the maximum value known for a ferromagnetic metal. Our first-principles calculations reveal that this arises from a significantly enhanced Berry curvature associated with Weyl points near the Fermi energy11. As this effect is geometrically convenient for thermoelectric power generation—it enables a lateral configuration of modules to cover a heat source6—these observations suggest that a new class of thermoelectric materials could be developed that exploit topological magnets to fabricate efficient, densely integrated thermopiles.

Charge-transport model for conducting polymers

Abstract: The growing technological importance of conducting polymers makes the fundamental understanding of their charge transport extremely important for materials and process design. Various hopping and mobility edge transport mechanisms have been proposed, but their experimental verification is limited to poor conductors. Now that advanced organic and polymer semiconductors have shown high conductivity approaching that of metals, the transport mechanism should be discernible by modelling the transport like a semiconductor with a transport edge and a transport parameter s. Here we analyse the electrical conductivity and Seebeck coefficient together and determine that most polymers (except possibly PEDOT:tosylate) have s = 3 and thermally activated conductivity, whereas s = 1 and itinerant conductivity is typically found in crystalline semiconductors and metals. The different transport in polymers may result from the percolation of charge carriers from conducting ordered regions through poorly conducting disordered regions, consistent with what has been expected from structural studies.

Significantly Enhanced Thermoelectric Properties of PEDOT:PSS Films through Sequential Post‐Treatments with Common Acids and Bases

Abstract: Thermoelectric (TE) materials are important for the sustainable development because they enable the direct harvesting of low-quality heat into electricity. Among them, conducting polymers have attracted great attention arising from their advantages, such as flexibility, nontoxicity, easy availability, and intrinsically low thermal conductivity. In this work, a novel and facile method is reported to significantly enhance the TE property of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films through sequential post-treatments with common acids and bases. Compared with the as-prepared PEDOT:PSS, both the Seebeck coefficients and electrical conductivities can be remarkably enhanced after the treatments. The oxidation level, which significantly impacts the TE property of the PEDOT:PSS films, can also be well tuned by controlling the experimental conditions during the base treatment. The optimal PEDOT:PSS films can have a Seebeck coefficient of 39.2 µV K−1 and a conductivity of 2170 S cm−1 at room temperature, and the corresponding power factor is 334 µW (m−1 K−2). The enhancement in the TE properties is attributed to the synergetic effect of high charge mobility by the acid treatment and the optimal oxidation level tuned by the base treatment.

Tuning the carrier scattering mechanism to effectively improve the thermoelectric properties

Abstract: A high thermoelectric power factor not only enables a potentially high figure of merit ZT but also leads to a large output power density, and hence it is pivotal to find an effective route to improve the power factor. Previous reports on the manipulation of carrier scattering mechanisms (e.g. ionization scattering) were mainly focused on enhancing the Seebeck coefficient. In contrast, here we demonstrate that by tuning the carrier scattering mechanism in n-type Mg3Sb2-based materials, it is possible to noticeably improve the Hall mobility, from ∼19 to ∼77 cm2 V−1 s−1, and hence substantially increase the power factor by a factor of 3, from ∼5 to ∼15 μW cm−1 K−2. The enhancement in mobility is mainly due to the reason that ionization scattering has been converted into mixed scattering between ionization and acoustic phonon scattering, which less effectively scatters the carriers. The strategy of tuning the carrier scattering mechanism to improve the mobility should be widely applicable to various material systems for achieving better thermoelectric performance.

Figure of merit ZT of a thermoelectric device defined from materials properties

Abstract: While the thermoelectric materials figure of merit is a well defined metric to evaluate thermoelectric materials, it can be a poor metric for maximum thermoelectric device efficiency because of the temperature dependence of the Seebeck coefficient S, the electrical resistivity ρ, and the thermal conductivity κ where T is the absolute temperature Historically the field has used a thermoelectric device figure of merit ZT to characterize a device operating between a hot side temperature Th and cold side temperature Tc While there are many approximate methods to calculate ZT from temperature dependent materials properties, an exact method is given here that uses a simple algorithm that can be performed on a spreadsheet calculator The figure of merit is defined for a thermoelectric generator using the maximum efficiency of the thermoelectric device η calculated from the exact method

Morphology controls the thermoelectric power factor of a doped semiconducting polymer

Abstract: The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with F n TCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (σ) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (μ). The Seebeck coefficient (α) is largely independent of OCL. This reveals that, unlike σ, leveraging strategies to improve μ have a smaller impact on α. Our best-performing sample with the largest OCL, vapor-doped PBTTT:F4TCNQ thin film, has a σ of 670 S/cm and an α of 42 μV/K, which translates to a large PF of 120 μW m-1 K-2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m-1 K-2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics.

Thermoelectric Properties of Two-Dimensional Molybdenum-Based MXenes

Abstract: MXenes are an interesting class of 2D materials consisting of transition metal carbides and nitrides, which are currently a subject of extensive studies. Although there have been theoretical calculations estimating the thermoelectric properties of MXenes, no experimental measurements have been reported so far. In this report, three compositions of Mo-based MXenes (Mo2CTx, Mo2TiC2Tx, and Mo2Ti2C3Tx) have been synthesized and processed into free-standing binder-free papers by vacuum-assisted filtration, and their electrical and thermoelectric properties are measured. Upon heating to 800 K, these MXene papers exhibit high conductivity and n-type Seebeck coefficient. The thermoelectric power reaches 3.09 × 10–4 W m–1 K–2 at 803 K for the Mo2TiC2Tx MXene. While the thermoelectric properties of MXenes do not reach that of the best materials, they exceed their parent ternary and quaternary layered carbides. Mo2TiC2Tx shows the highest electrical conductivity in combination with the largest Seebeck coefficient of...

Promising Thermoelectric Bulk Materials with 2D Structures

Abstract: Given that more than two thirds of all energy is lost, mostly as waste heat, in utilization processes worldwide, thermoelectric materials, which can directly convert waste heat to electricity, provide an alternative option for optimizing energy utilization processes. After the prediction that superlattices may show high thermoelectric performance, various methods based on quantum effects and superlattice theory have been adopted to analyze bulk materials, leading to the rapid development of thermoelectric materials. Bulk materials with two-dimensional (2D) structures show outstanding properties, and their high performance originates from both their low thermal conductivity and high Seebeck coefficient due to their strong anisotropic features. Here, the advantages of superlattices for enhancing the thermoelectric performance, the transport mechanism in bulk materials with 2D structures, and optimization methods are discussed. The phenomenological transport mechanism in these materials indicates that thermal conductivities are reduced in 2D materials with intrinsically short mean free paths. Recent progress in the transport mechanisms of Bi2 Te3 -, SnSe-, and BiCuSeO-based systems is summarized. Finally, possible research directions to enhance the thermoelectric performance of bulk materials with 2D structures are briefly considered.

Electronic origin of the high thermoelectric performance of GeTe among the p-type group IV monotellurides

Abstract: PbTe and SnTe in their p-type forms have long been considered high-performance thermoelectrics, and both of them largely rely on two valence bands (the first band at L point and the second one along the Σ line) participating in the transport properties. This work focuses on the thermoelectric transport properties inherent to p-type GeTe, a member of the group IV monotellurides that is relatively less studied. Approximately 50 GeTe samples have been synthesized with different carrier concentrations spanning from 1 to 20 × 1020 cm−3, enabling an insightful understanding of the electronic transport and a full carrier concentration optimization for the thermoelectric performance. When all of these three monotellurides (PbTe, SnTe and GeTe) are fully optimized in their p-type forms, GeTe shows the highest thermoelectric figure of merit (zT up to 1.8). This is due to its superior electronic performance, originating from the highly degenerated Σ band at the band edge in the low-temperature rhombohedral phase and the smallest effective masses for both the L and Σ bands in the high-temperature cubic phase. The high thermoelectric performance of GeTe that is induced by its unique electronic structure not only provides a reference substance for understanding existing research on GeTe but also opens new possibilities for the further improvement of the thermoelectric performance of this material. A team has uncovered the electronic reasons why germanium tellurides (GeTe) are superior energy harvesters to their periodic counterparts. Efforts to improve GeTe thermoelectric efficiency focus mostly on reducing heat conductivity inside its phase-changing crystal lattice. Yanzhong Pei from Tongji University in Shanghai, China, and colleagues now show how semiconductor band tuning may also improve this compound. They synthesized over 50 samples of GeTe with a broad range of carrier concentrations. The researchers then compared transport measurements and theoretical band calculations with those for the more common lead and tin telluride thermoelectrics. The GeTe samples with the best conversion efficiency had charge carriers with lower effective masses than usual and access to numerous energy bands — a combination that enables more heat to be converted to electricity compared to lead telluride or tin telluride. This work focuses on the thermoelectric properties inherent to p-type GeTe by tuning the carrier concentration. Compared with PbTe and SnTe, GeTe shows the highest Seebeck coefficient (a) and power factor (b) in the important carrier concentration range of 1–3 × 1020 cm−3. Provided all are fully optimized in carrier concentrations, GeTe shows the highest thermoelectric figure of merit, zT at all temperatures (c). This originates from its highest degeneracy of the first valence band (along Σ line) in the low temperature phase, and the smallest effective masses for both L and Σ bands in the high temperature phase (d).

High thermoelectric power factor in two-dimensional crystals of Mo S 2

Abstract: The quest for high-efficiency heat-to-electricity conversion has been one of the major driving forces toward renewable energy production for the future. Efficient thermoelectric devices require high voltage generation from a temperature gradient and a large electrical conductivity while maintaining a low thermal conductivity. For a given thermal conductivity and temperature, the thermoelectric power factor is determined by the electronic structure of the material. Low dimensionality (1D and 2D) opens new routes to a high power factor due to the unique density of states (DOS) of confined electrons and holes. The 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only due to such confinement effects but especially due to their large effective masses and valley degeneracies. Here, we report a power factor of $\mathrm{Mo}{\mathrm{S}}_{2}$ as large as $8.5\phantom{\rule{0.28em}{0ex}}\mathrm{mW}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$ at room temperature, which is among the highest measured in traditional, gapped thermoelectric materials. To obtain these high power factors, we perform thermoelectric measurements on few-layer $\mathrm{Mo}{\mathrm{S}}_{2}$ in the metallic regime, which allows us to access the 2D DOS near the conduction band edge and exploit the effect of 2D confinement on electron scattering rates, resulting in a large Seebeck coefficient. The demonstrated high, electronically modulated power factor in 2D TMDCs holds promise for efficient thermoelectric energy conversion.

Extraordinary Thermoelectric Performance Realized in n-Type PbTe through Multiphase Nanostructure Engineering.

Abstract: Lead telluride has long been realized as an ideal p-type thermoelectric material at an intermediate temperature range; however, its commercial applications are largely restricted by its n-type counterpart that exhibits relatively inferior thermoelectric performance. This major limitation is largely solved here, where it is reported that a record-high ZT value of ≈1.83 can be achieved at 773 K in n-type PbTe-4%InSb composites. This significant enhancement in thermoelectric performance is attributed to the incorporation of InSb into the PbTe matrix resulting in multiphase nanostructures that can simultaneously modulate the electrical and thermal transport. On one hand, the multiphase energy barriers between nanophases and matrix can boost the power factor in the entire temperature range via significant enhancement of the Seebeck coefficient and moderately reducing the carrier mobility. On the other hand, the strengthened interface scattering at the intensive phase boundaries yields an extremely low lattice thermal conductivity. This strategy of constructing multiphase nanostructures can also be highly applicable in enhancing the performance of other state-of-the-art thermoelectric systems.

Wide bandgap BaSnO3 films with room temperature conductivity exceeding 104 S cm-1.

Abstract: Wide bandgap perovskite oxides with high room temperature conductivities and structural compatibility with a diverse family of organic/inorganic perovskite materials are of significant interest as transparent conductors and as active components in power electronics. Such materials must also possess high room temperature mobility to minimize power consumption and to enable high-frequency applications. Here, we report n-type BaSnO3 films grown using hybrid molecular beam epitaxy with room temperature conductivity exceeding 104 S cm-1. Significantly, these films show room temperature mobilities up to 120 cm2 V-1 s-1 even at carrier concentrations above 3 × 1020 cm-3 together with a wide bandgap (3 eV). We examine the mobility-limiting scattering mechanisms by calculating temperature-dependent mobility, and Seebeck coefficient using the Boltzmann transport framework and ab-initio calculations. These results place perovskite oxide semiconductors for the first time on par with the highly successful III-N system, thereby bringing all-transparent, high-power oxide electronics operating at room temperature a step closer to reality.

Large enhancement of thermoelectric properties in n-type PbTe via dual-site point defects

Abstract: The relatively inferior performance of n-type legs impedes the application of PbTe materials in intermediate temperature thermoelectric devices. In order to elevate the thermoelectric properties of n-type PbTe, we added some Sb phases into 0.1% PbI2-doped PbTe by a conventional melting method. Transmission electron microscopy (TEM) analysis together with density function theory (DFT) calculations showed that endotaxial Sb nanoprecipitates were produced in the PbTe samples at room temperature, and that part of these nanoprecipitates gradually dissolved into the PbTe matrix to form SbPb–SbTe dual-site substitutional point defects as temperature increased. A maximum ZT of about 1.8 was achieved at 773 K for the n-type PbTe0.998I0.002–3%Sb composite due to a simultaneous improvement in power factor and reduction in lattice thermal conductivity. In the PbTe0.998I0.002–x%Sb (x = 1–4) composite samples, the Seebeck coefficient was much higher than that of the reported single-phase PbTe samples with similar carrier concentration, which mainly originated from distortion of the density-of-states caused by Sb dual-site doping. Simulations based on the Callaway model suggested that the SbPb–SbTe dual-site substitutional point defects also played an important role in decreasing the lattice thermal conductivity at elevated temperature. We propose that the synergistic role of Sb in both electrical and thermal transport should be highly applicable in other bulk thermoelectric materials.

Resonant level-induced high thermoelectric response in indium-doped GeTe

Abstract: Resonant levels are promising for high-performance single-phase thermoelectric materials. Recently, phase-change materials have attracted much attention for energy conversion applications. As the energetic position of resonant levels could be temperature dependent, searching for dopants in phase-change materials, which can introduce resonant levels in both low and high temperature phases, remains challenging. In this study, possible distortions of the electronic density of states due to group IIIA elements (Ga, In, Tl) in GeTe are theoretically investigated. Resonant levels induced by indium dopants in both rhombohedral and cubic phase GeTe have been demonstrated. The experimental Seebeck coefficients of InxGe1−xTe exhibit a large enhancement compared with those observed for other prior dopants. Indium dopants reduce the defect concentrations in GeTe, and thus, they lower the carrier concentrations and suppress the electronic component of the total thermal conductivity. The enhanced Seebeck coefficient, together with the suppressed thermal conductivity, leads to a reasonably high ZT of 1.3 at a temperature near 355 °C in In0.02Ge0.98Te. The corresponding average ZT is enhanced by ~70% across the entire temperature range of the rhombohedral and cubic phases. These observations indicate that indium-doped GeTe is a promising base material for achieving an even higher thermoelectric performance. The promising heat-to-electricity conversion efficiency of phase-change materials can be enhanced with indium dopants. Germanium telluride (GeTe) has a high thermoelectric response because it stores latent heat through temperature-dependent changes to its crystal phase. Now, researchers in China and the USA have used quantum simulations to investigate electronic routes to further boost energy harvesting by GeTe. Their computations searched through numerous chemical elements and found that adding small quantities of indium atoms opens up new resonant energy levels that could lead to extra heat retention in both of GeTe's phases. Experimental synthesis of indium-doped GeTe demonstrated the validity of this approach, with optimized samples showing a 70% improvement in thermoelectric conversion across a wide temperature range. In both the rhombohedral and the cubic phase GeTe, resonant levels induced by indium dopants have been demonstrated. The experimental Seebeck coefficients of InxGe1−xTe show large enhancement as compared with other prior dopants. The enhanced Seebeck coefficient, combined with the reduced thermal conductivity, leads to a reasonably high ZT of 1.3 near 355 °C in In0.02Ge0.98Te. The average ZT is enhanced by ~70% across the whole temperature range. The present results suggest that indium-doped GeTe can be a promising base-material for even higher thermoelectric performance.

Bottom-up engineering of thermoelectric nanomaterials and devices from solution-processed nanoparticle building blocks

Abstract: The conversion of thermal energy to electricity and vice versa by means of solid state thermoelectric devices is extremely appealing. However, its cost-effectiveness is seriously hampered by the relatively high production cost and low efficiency of current thermoelectric materials and devices. To overcome present challenges and enable a successful deployment of thermoelectric systems in their wide application range, materials with significantly improved performance need to be developed. Nanostructuration can help in several ways to reach the very particular group of properties required to achieve high thermoelectric performances. Nanodomains inserted within a crystalline matrix can provide large charge carrier concentrations without strongly influencing their mobility, thus allowing to reach very high electrical conductivities. Nanostructured materials contain numerous grain boundaries that efficiently scatter mid- and long-wavelength phonons thus reducing the thermal conductivity. Furthermore, nanocrystalline domains can enhance the Seebeck coefficient by modifying the density of states and/or providing type- and energy-dependent charge carrier scattering. All these advantages can only be reached when engineering a complex type of material, nanocomposites, with exquisite control over structural and chemical parameters at multiple length scales. Since current conventional nanomaterial production technologies lack such level of control, alternative strategies need to be developed and adjusted to the specifics of the field. A particularly suitable approach to produce nanocomposites with unique level of control over their structural and compositional parameters is their bottom-up engineering from solution-processed nanoparticles. In this work, we review the state-of-the-art of this technology applied to the thermoelectric field, including the synthesis of nanoparticles of suitable materials with precisely engineered composition and surface chemistry, their combination and consolidation into nanostructured materials, the strategies to electronically dope such materials and the attempts to fabricate thermoelectric devices using nanoparticle-based nanopowders and inks.

Large n- and p-type thermoelectric power factors from doped semiconducting single-walled carbon nanotube thin films

Abstract: Lightweight, robust, and flexible single-walled carbon nanotube (SWCNT) materials can be processed inexpensively using solution-based techniques, similar to other organic semiconductors. In contrast to many semiconducting polymers, semiconducting SWCNTs (s-SWCNTs) represent unique one-dimensional organic semiconductors with chemical and physical properties that facilitate equivalent transport of electrons and holes. These factors have driven increasing attention to employing s-SWCNTs for electronic and energy harvesting applications, including thermoelectric (TE) generators. Here we demonstrate a combination of ink chemistry, solid-state polymer removal, and charge-transfer doping strategies that enable unprecedented n-type and p-type TE power factors, in the range of 700 μW m−1 K−2 at 298 K for the same solution-processed highly enriched thin films containing 100% s-SWCNTs. We also demonstrate that the thermal conductivity appears to decrease with decreasing s-SWCNT diameter, leading to a peak material zT ≈ 0.12 for s-SWCNTs with diameters in the range of 1.0 nm. Our results indicate that the TE performance of s-SWCNT-only material systems is approaching that of traditional inorganic semiconductors, paving the way for these materials to be used as the primary components for efficient, all-organic TE generators.

Boosting the Thermoelectric Performance of (Na,K)-Codoped Polycrystalline SnSe by Synergistic Tailoring of the Band Structure and Atomic-Scale Defect Phonon Scattering

Abstract: We report the high thermoelectric performance of p-type polycrystalline SnSe obtained by the synergistic tailoring of band structures and atomic-scale defect phonon scattering through (Na,K)-codoping. The energy offsets of multiple valence bands in SnSe are decreased after Na doping and further reduced by (Na,K)-codoping, resulting in an enhancement in the Seebeck coefficient and an increase in the power factor to 492 μW m–1 K–2. The lattice thermal conductivity of polycrystalline SnSe is decreased by the introduction of effective phonon scattering centers, such as point defects and antiphase boundaries. The lattice thermal conductivity of the material is reduced to values as low as 0.29 W m–1 K–1 at 773 K, whereas ZT is increased from 0.3 for 1% Na-doped SnSe to 1.2 for 1% (Na,K)-codoped SnSe.

Effective mass and Fermi surface complexity factor from ab initio band structure calculations

Abstract: The effective mass is a convenient descriptor of the electronic band structure used to characterize the density of states and electron transport based on a free electron model. While effective mass is an excellent first-order descriptor in real systems, the exact value can have several definitions, each of which describe a different aspect of electron transport. Here we use Boltzmann transport calculations applied to ab initio band structures to extract a density-of-states effective mass from the Seebeck Coefficient and an inertial mass from the electrical conductivity to characterize the band structure irrespective of the exact scattering mechanism. We identify a Fermi Surface Complexity Factor: $${N}_{{\rm{v}}}^{\ast }{K}^{\ast }$$ from the ratio of these two masses, which in simple cases depends on the number of Fermi surface pockets $$({N}_{{\rm{v}}}^{\ast })$$ and their anisotropy K *, both of which are beneficial to high thermoelectric performance as exemplified by the high values found in PbTe. The Fermi Surface Complexity factor can be used in high-throughput search of promising thermoelectric materials. A simple method for determining a material’s thermoelectric properties is developed by researchers in the United States and Belgium. Jeffrey Snyder from Northwestern University and his co-workers’ model could simplify the search for materials that efficiently generate electricity from waste heat. Even though the environment of an electron in a solid is very complex, the way an electron moves through a solid’s lattice of atoms can be treated as if it is moving in free space. However, because of the influence of its environment an effective mass, not its true mass, is used to model the movement of electrons and that material’s properties. But this effective-mass can be defined in several ways depending on which material property is being modeled. Snyder et al. determine that the ratio of two different effective masses, as computed from different electronic properties, could be a good method to identify novel thermoelectric materials and can be associated with the “complexity” of the electronic structure.

High Thermoelectric Performance in Electron-Doped AgBi3S5 with Ultralow Thermal Conductivity

Abstract: We report electron-doped AgBi3S5 as a new high-performance nontoxic thermoelectric material. This compound features exceptionally low lattice thermal conductivities of 0.5–0.3 W m–1 K–1 in the temperature range of 300–800 K, which is ascribed to its unusual vibrational properties: “double rattling” phonon modes associated with Ag and Bi atoms. Chlorine doping at anion sites acts as an efficient electron donor, significantly enhancing the electrical properties of AgBi3S5. In the carrier concentration range (5 × 1018–2 × 1019 cm–3) investigated in this study, the trends in Seebeck coefficient can be reasonably understood using a single parabolic band model with the electron effective mass of 0.22 me (me is the free electron mass). Samples of 0.33% Cl-doped AgBi3S5 prepared by spark plasma sintering show a thermoelectric figure of merit of ∼1.0 at 800 K.

The bridge between the materials and devices of thermoelectric power generators

Abstract: While considerable efforts have been made to develop and improve thermoelectric materials, research on thermoelectric modules is at a relatively early stage because of the gap between material and device technologies. In this review, we discuss the cumulative temperature dependence model to reliably predict the thermoelectric performance of module devices and individual materials for an accurate evaluation of the p–n configuration compared to the conventional model used since the 1950s. In this model, the engineering figure of merit and engineering power factor are direct indicators, and they exhibit linear correlations to efficiency and output power density, respectively. To reconcile the strategy for high material performance and the thermomechanical reliability issue in devices, a new methodology is introduced by defining the engineering thermal conductivity. Beyond thermoelectric materials, the device point of view needs to be actively addressed before thermoelectric generators can be envisioned as power sources.

High Conductivity and Electron-Transfer Validation in an n-Type Fluoride-Anion-Doped Polymer for Thermoelectrics in Air.

Abstract: Air-stable and soluble tetrabutylammonium fluoride (TBAF) is demonstrated as an efficient n-type dopant for the conjugated polymer ClBDPPV. Electron transfer from F− anions to the π-electron-deficient ClBDPPV through anion–π electronic interactions is strongly corroborated by the combined results of electron spin resonance, UV–vis–NIR, and ultraviolet photoelectron spectroscopy. Doping of ClBDPPV with 25 mol% TBAF boosts electrical conductivity to up to 0.62 S cm−1, among the highest conductivities that have been reported for solution-processed n-type conjugated polymers, with a thermoelectric power factor of 0.63 µW m−1 K−2 in air. Importantly, the Seebeck coefficient agrees with recently published correlations to conductivity. Moreover, the F−-doped ClBDPPV shows significant air stability, maintaining the conductivity of over 0.1 S cm−1 in a thick film after exposure to air for one week, to the best of our knowledge the first report of an air-stable solution-processable n-doped conductive polymer with this level of conductivity. The result shows that using solution-processable small-anion salts such as TBAF as an n-dopant of organic conjugated polymers possessing lower LUMO (lowest unoccupied molecular orbital), less than −4.2 eV) can open new opportunities toward high-performance air-stable solution-processable n-type thermoelectric (TE) conjugated polymers.

Thermoelectric properties of CuGa1−xMnxTe2: power factor enhancement by incorporation of magnetic ions

Abstract: Chalcopyrite CuGaTe2 is under research for its high thermoelectric performance. Different routes have been investigated recently for enhancing its thermoelectric parameters. In this work we report the synthesis of chalcopyrite CuGa1−xMnxTe2 (x = 0.0, 0.01, 0.02, and 0.03) by a solid state method and through which an enhanced power factor was obtained. The samples were characterized for electrical, thermal and thermoelectric transport properties in the temperature range 325–870 K after performing stability analysis using TG-DTA data. XRD patterns confirm a phase pure tetragonal structure for all nominal compositions with the space group I2d. The electrical conductivity σ increases drastically by Mn2+ doping which increases the hole carriers, while the Seebeck coefficient S still retains large positive values. As a result, the power factor of CuGa0.99Mn0.01Te2 reaches 1.55 mW K−2 m−1 at 718 K. Calculations using the relationship of S and ln σ suggest that the power factor observed for Mn-doped samples is higher than that expected for CuGaTe2 with optimized carrier concentration, suggesting that the Mn-doping brings additional effects other than simple carrier tuning. The total thermal conductivity is reduced by Mn doping, with a minimum thermal conductivity of 1.6 W m−1 K−1 for the x = 0.01 sample. The maximum value for ZT reached at 870 K was 0.83, which is more than 40% enhancement as compared to that of pure CuGaTe2. Strong interactions between the magnetic moments of Mn and charge carriers are inferred by the large negative Weiss temperature in the magnetic susceptibility and distinct anomalous Hall effect, the latter of which develops in accordance with the increase of magnetization at low temperature. These results suggest that the carrier–magnetic moment interaction plays an essential role in the enhancement of the thermoelectric properties of CuGa1−xMnxTe2.

A solution-processed TiS2/organic hybrid superlattice film towards flexible thermoelectric devices

Abstract: Liquid-exfoliation has proven to be a scalable and versatile technique to produce high-yield two-dimensional nanosheets in graphene, BN, layered perovskites and transition metal dichalcogenides. This also provides new insights into the construction of novel nanoelectronics and nanophotonics through the assembly of nanosheets. Here we present a simple exfoliation-and-reassembly approach to produce a flexible n-type TiS2/organic hybrid film for low-temperature thermoelectric applications. The obtained film shows a superlattice structure with alternative layers of TiS2 and organic molecules. Charge transfer occurs when TiS2 and organic molecules form intercalation complexes, which gives rise to a high electrical conductivity but a low Seebeck coefficient. However, the power factor can be further enhanced by annealing the film under vacuum, and the value reaches 210 μW m−1 K−2 at room temperature in this study. Our flexible thermoelectric device can generate a high power density of 2.5 W m−2 at a temperature gradient of 70 K, which hits a new record among organic-based thermoelectric devices.

Exploring High-Performance n-Type Thermoelectric Composites Using Amino-Substituted Rylene Dimides and Carbon Nanotubes.

Abstract: Taking advantage of the high electrical conductivity of a single-walled carbon nanotube (SWCNT) and the large Seebeck coefficient of rylene diimide, a convenient strategy is proposed to achieve high-performance n-type thermoelectric (TE) composites containing a SWCNT and amino-substituted perylene diimide (PDINE) or naphthalene diimide (NDINE) The obtained n-type composites display greatly enhanced TE performance with maximum power factors of 112 ± 8 (PDINE/SWCNT) and 135 ± 14 (NDINE/SWCNT) μW m–1 K–2 A short doping time of 05 h can ensure high TE performance The corresponding TE module consisting of five p–n junctions reaches a large output power of 33 μW under a 50 °C temperature gradient In addition, the n-type composites exhibit high air stability and excellent thermal stability This design strategy benefits the future fabricating of high-performance n-type TE materials and devices

Two-dimensional InSe as a potential thermoelectric material

Abstract: Thermoelectric properties of monolayer indium selenide (InSe) are investigated by using Boltzmann transport theory and first-principles calculations as a function of Fermi energy and crystal orientation. We find that the maximum power factor of p-type (n-type) monolayer InSe can be as large as 0.049 (0.043) W/K2m at 300 K in the armchair direction. The excellent thermoelectric performance of monolayer InSe is attributed to both its Seebeck coefficient and electrical conductivity. The large Seebeck coefficient originates from the moderate (about 2 eV) bandgap of monolayer InSe as an indirect gap semiconductor, while its large electrical conductivity is due to its unique two-dimensional density of states (DOS), which consists of an almost constant DOS near the conduction band bottom and a sharp peak near the valence band top.

Enhancement of graphene thermoelectric performance through defect engineering

Abstract: Thermoelectric properties of materials are typically evaluated using the figure of merit, ZT, which relies on both the electrical and thermal properties of the materials. Although graphene has a high thermoelectric power factor, its overall ZT value is quite low as it possesses extremely high thermal conductivity. Phonons are the main heat carrier in graphene, and therefore propagation of heat in the material may be modulated by introducing defects into the structure, resulting in reduced thermal conductivity. In this study, we investigate the effect of graphene defect density on the thermoelectric performance of graphene. The defects introduced into graphene by oxygen plasma treatment reduce its Seebeck coefficient as well as its electrical conductivity; as a result, the thermoelectric power factor declines with increasing defect density. However, at higher defect densities, the reduction in thermal conductivity dominates over the reduction in electrical conductivity and, consequently, graphene treated in this way is observed to possess ZT values of up to three times that of pristine graphene. Therefore, it may be concluded that introducing controlled amount of defects into graphene is an effective way of reducing its thermal conductivity, thereby enhancing the performance of graphene-based thermoelectric devices.

Enhancing thermoelectric performance in hierarchically structured BiCuSeO by increasing bond covalency and weakening carrier–phonon coupling

Abstract: BiCuSeO oxyselenides are promising thermoelectric materials at intermediate temperatures, primarily due to their ultralow lattice thermal conductivity (κL) and high Seebeck coefficient. The intrinsically low carrier mobility in these materials, normally below ∼20 cm2 V−1 s−1 at 300 K, however, largely limits further improvements of their thermoelectric properties. In this study, by introducing less electronegative Te into the conductive Cu–Se layers, we demonstrate that the enhanced chemical bond covalency results in smaller effective mass and thus improved carrier mobility, through the weakening of carrier-phonon coupling. The improved carrier mobility by Te-doping largely retains the electrical conductivity values and thus high power factors even with decreased carrier concentrations. Meanwhile, the hierarchical structural features including dual point defects, nanoinclusions, grain boundaries, etc., originating from the nonequilibrium self-propagating high-temperature synthesis (SHS) processes, further reduce κL close to the amorphous limit. Ultimately, a maximum ZT value of ∼1.2 at 873 K is achieved in Bi0.96Pb0.04CuSe0.95Te0.05O, ∼35% improvement as compared with that of Te-free Bi0.96Pb0.04CuSeO and ∼2.4 times higher than that of the pristine sample. Furthermore, our study elucidates that weakening of carrier–phonon coupling through regulating chemical bonding within the conductive functionalities can be an effective avenue for further improving the thermoelectric performance of BiCuSeO.